1. Field of the Invention
The present invention relates to a crystal pulling method and apparatus employing the Czochralski method (the CZ method), and particularly to improvements for safe manufacture of heavy single crystals of high quality.
2. Description of the Related Art
Regarding a method of pulling a single crystal from a melt of a semiconductor material, such as silicon, contained in a crucible, the present inventors have proposed an improved pulling technique as described in, for example, Japanese Patent Application No. 7-256892.
As shown in FIG. 6, according to this technique for pulling a single crystal, a manufacturing method for growing a single crystal portion 55, comprising the steps of bringing a seed crystal 52 held by a seed chuck 51 into contact with a material melt 53 and pulling the seed chuck 51 by a seed chuck lifting mechanism 54, is performed through use of an apparatus having the following structure.
A retaining case 58 rotatable about a vertical axis is mounted on a slider 57 which is vertically moved by a slider moving mechanism 56. The seed chuck lifting mechanism 54 is provided in the retaining case 58 so as to pull the seed chuck 51 by means of a wire 60. The retaining case 58 is equipped with lifting holders 61 which have engagement portions 61a at the bottom end thereof. Thus, the single crystal portion 55 is pulled through use of both the seed chuck 51 and the lifting holders 61.
That is, initially, with the slider 57 fixed at a certain position, the seed crystal 52 held by the seed chuck 51 is brought into contact with the material melt 53, and subsequently the seed chuck 51 is pulled by the seed chuck lifting mechanism 54 while the retaining case 58 is being rotated, to thereby form a neck portion 62, a corrugated portion 63, and the single crystal portion 55, in this order subsequent to the seed crystal 52. Then, when the corrugated portion 63 rises to a predetermined position, the lifting holders 61 operate so that the engagement portions 61a engage a concave section of the corrugated portion 63. Subsequently, the slider moving mechanism 56 operates to start raising the slider 57.
At this time, control is performed such that the sum of the rising speed of the slider 57 and the rising speed of the seed chuck 51 relative to the slider 57 becomes identical to the previous rising speed of the seed chuck 51 as measured when solely the seed chuck 51 was engaged in pulling. The rising speed of the slider 57 gradually increases, and accordingly that of the seed chuck 51 gradually decreases. Finally, the rising speed of the seed chuck 51 relative to the slider 57 becomes zero, after which the pulling is completely performed by the lifting holders 61.
Such a pulling method allows the safe and reliable growth of the single crystal portion 55 with no breakage of the neck portion 62 even when the diameter and weight of the single crystal portion 55 are increased.
However, in the above-described technique, since the lifting holders 61 approach the corrugated portion 63 in a lateral direction to hold it, a lateral force is likely to act on the corrugated portion 63, causing the single crystal portion 55 to shake with a resultant occurrence of dislocation therein.
Also, when the lifting holders 61 completely replace the seed chuck 51 as pulling means, and a load imposed on the seed chuck 51 becomes zero, the wire used to pull the seed chuck 51 is likely to slacken, potentially damaging the neck portion 62.
In a conventional crystal pulling apparatus having a single pulling mechanism, a detected length of pulling by the pulling mechanism indicates a vertical position of the seed chuck 51 as well as the length of a grown single crystal. By contrast, in the above-described technique involving two pulling mechanisms, the respective lengths of pulling by the two pulling mechanisms must be added. Also, when the wire slackens, it is difficult to accurately measure the length of pulling by the seed chuck 51. Thus, the sum of these measurements is likely to involve an error.
Further, when the pulling by the seed chuck 51 is switched to the pulling by the lifting holders 61, the pulling speed of the seed chuck 51 is varied so as to control the diameter. Therefore, operating conditions such as time required to switch the pulling by the seed chuck 51 to the pulling by the lifting holders 61 are not stable.
Under the above circumstances, there has been eager demand for means which allows the lifting holders 61 to gently hold the corrugated portion without applying a lateral force thereto, which maintains the wire used to pull the seed chuck 51 in a taut state even after the pulling means is switched from the seed chuck 51 to the pulling by the lifting holders 61, and which can accurately measure the length of a grown single crystal.